A drug carrier, wherein a drug is carried on a vector (carrier), is attracting attention as a drug delivery system (hereinafter to be described as “DDS”) to certainly, safely and efficiently deliver a drug into the cells of the object lesion site, and has been actively studied. Particularly, when the drug is a physiologically active substance that expresses activity in cells, such as protein, DNA (gene), antisense molecule and the like, the drug needs to be introduced into the cytoplasm to show effective action. However, since cell membrane does not allow permeation of many water-soluble substances and polymers, a vector (carrier) to deliver the drug into the cytoplasm is necessary. Vector includes virus vector and synthetic vector. Since virus vector has safety problems, the development of a safe and highly functional synthetic vector is desired, and use of lipid membrane vesicles such as liposome, emulsion, lipid microsphere and the like has been considered.
Particularly, as the above-mentioned synthetic vector, studies have been made to apply vesicles such as liposome and the like to DDS as a drug carrier. These vesicles are generally prepared from phospholipid or a derivative thereof, sterol, lipid other than phospholipid, and the like as a basic membrane constituent. With these basic constituent components alone, however, problems of coagulation of vesicles, low retention in the body and the like are difficult to solve, and delivery of the drug contained in the liposome to the object site (for example, target organ and cell) as a DDS preparation is difficult. To solve these problems, it has been reported to avoid interaction with blood components, blood cells and vascular endothelial cells by forming a liposome using a polyglycerol phospholipid derivative (for example, patent document 1). However, although this technique maintains retention in blood, the presence of a water-soluble polymer becomes an obstacle that prevents the drug from approaching and contacting the organ or cell to act on, and intracellular introduction rate of the drug becomes problematically low.
Therefore, studies of enhanced introduction into the cell by modification of a liposome membrane with a substance showing an interaction with a biomembrane are also ongoing. For example, a succinylated derivative of polyglycerol (hereinafter to be indicated as SucPG), which shows membrane fusion under mild acidic conditions, has been reported (for example, patent document 2). In addition, it has been reported that a drug carrier wherein a drug is carried on carrier particles of lipid membrane modified by a 3-methylglutaric acid derivative of polyglycerol which has different hydrocarbon chain length from SucPG and a branched structure, and an amidine derivative which is a cationation modifier, has a transgene activity (for example, patent document 3). However, in these derivatives, a hydrocarbon group is introduced into polyglycerol at not desired positions but random positions. Therefore, when these derivatives are used as constituent substances of carrier particles made of lipid membranes such as liposome, the hydrocarbon group to be the anchor part is randomly incorporated into the lipid membrane at plural positions. As a result, hydrophilic polyglycerol chain is restricted and cannot separate much from the liposome surface, thus failing to expand the hydration sphere. Therefore, a stable liposome is difficult to form.